1. Field of the Invention
This invention relates to a pressure sensor which detects a variation in the pressure in an atmosphere and/or liquid by the indication of an electrical output signal.
2. Description of the Prior Art
The measurement of pressure in an atmosphere or liquid has been carried out by mechanical variations of Bourdon's tubes, bellows, diaphragms, etc., which are popular therefor since they are inexpensive and not troublesome. However, the development of pressure sensors, which will be able to be adapted to an advanced electronic technology, thereby allowing for the measurement of a variation in the external pressure by the indication of an electrical output signal, is most desirable. Such pressure sensors can be connected to a data processing system, thereby attaining automatic measurement of pressure and automatic control of the sensors. Moreover, the use of an electrical output signal for the measurement of a pressure variation allows not only extremely precise detection of the pressure variation, but also rapid response. It also allows for the miniaturization of the size and the weight of the pressure sensors. Thus, a variety of pressure sensors have been developed, examples of which are as follows:
(1) A pressure sensor, which is composed of a metal diaphragm having a strain gauge of a metal foil, can detect a variation in the external pressure by the deformation of the diaphragm due to a variation in the pressure causing a strain of the metal foil resulting in a variation in the electrical resistance of the metal foil. This sensor in inferior in that it is less pressure-sensitive and can not be manufactured with a miniaturized size and weight, although it is superior in that it can be used under a high pressure and it has an excellent temperature characteristic which is stable for a long period.
(2) A Si diaphragm-type pressure sensor can detect a variation in the external pressure by the piezoresistance effect which results from a variation in the specific resistance of a Si crystal due to the application of pressure to the Si crystal. Although this sensor has advantages in that, since silicon is used as an element therefor, semiconductor technology can be applied thereto which allows mass production thereof and an incorporation with external circuits required therefor, the sensitivity depends greatly upon temperature and thus a circuit for the compensation of temperature is essential thereto. Although a pressure sensor incorporating both a temperature-compensating circuit and a Si diaphragmtype sensor onto the same Si-substrate has been proposed, it is expensive and, moreover, liable to breakage because of a lack of mechanical strength of the Si diaphragm.
(3) A presure sensor, containing piezoelectric elements made of polyvinylidenefluoride, zinc oxides, etc., therein, can detect a variation in the external pressure by the piezoelectric effect, which results from a strain of the piezoelectric elements due to the pressure to thereby produce an electromotive force. Although this sensor is excellent in that it can be small, lightweight and produce a high output power, it is greatly influenced by noise due to external vibrations.
(4) A pressure sensor using a quartz oscillator can detect a variation in the external pressure by utilizing a linear relationship between the oscillation frequency of the quartz and the pressure variation to be measured. This sensor is expensive and cannot be manufactured with a miniaturized size and weight.
(5) Other pressure sensors which detect a variation in the external pressure by a variation in the electrical capacitance based on a displacement of the diaphragm therein due to a variation in the pressure have been developed. These sensors are expensive and cannot be manufactured with a miniaturized size and weight.
(6) Other pressure sensors which detect a variation in the external pressure by a variation in the inductance based on the magnetiostrictive effect, which results from a stress of the magnet therein due to the pressure have been developed. These sensors are also expensive and cannot be manufactured with a miniaturized size and weight.
As mentioned above, pressure sensors which have been already developed are either inferior in characteristics or expensive, so that they cannot be put in practical use.